1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating the same. More particularly, the invention relates to a bipolar transistor.
2. Description of the Background Art
FIGS. 11 to 13 are cross-sectional views showing a method of fabricating a conventional bipolar transistor in sequence of fabrication.
There is initially provided a p type silicon substrate 1 having a low impurity concentration. A high-concentration n type collector buried layer 2 and p type buried layers 3 are formed on the silicon substrate 1. A low-concentration n.sup.- type epitaxial layer 4 is grown on the silicon substrate 1 and buried layers 2 and 3. Oxide and nitride films (not shown) are deposited on the epitaxial layer 4, and the nitride film is left behind only in a region in which a bipolar transistor is to be formed (hereinafter referred to as an "active region"). Channel-cut high-concentration p type impurity ions are implanted by using a resist mask (not shown). Thick oxide films 110 for isolation are formed by selective oxidation, using the nitride film as a mask. At the same time, p type channel-cut layers 5 are formed. The isolation of the transistor to be formed later is achieved by both the p type buried layers 3 and the p type channel-cut layers 5. The epitaxial layer 4 is selectively exposed, using a resist mask (not shown). Ion implantation of high-concentration n type impurities and heat treatment are carried out to form a collector wall layer 6. The surface of the epitaxial layer 4 is selectively exposed in the active region. A polysilicon film is formed over the top surface, and high-concentration p type impurity ions are implanted into the polysilicon film. The polysilicon film is patterned by using a resist mask (not shown), so that an ion-implanted polysilicon film 200 is left behind. An oxide film 120 is then deposited over the top surface at low temperatures (FIG. 11).
The oxide film 120 and polysilicon film 200 are selectively etched off using a resist mask (not shown) to expose the surface of the epitaxial layer 4. The selectively remaining polysilicon film 200 becomes external base electrodes 201. The exposed surface of the epitaxial layer 4 is oxidized to form a thin oxide film 130. Ions of p type impurity for intrinsic base are implanted through the oxide film 130 to form a p type impurity implanted layer 7a (FIG. 12).
An oxide film is deposited over the top surface and is dry etched all over, so that a side wall oxide film 140 is formed only on the end face of the base electrode 201 and the impurity implanted layer 7a is exposed. A polysilicon film is deposited over the top surface. High-concentration n type impurity ions are implanted into the polysilicon film, which is in turn patterned to form an emitter electrode 210. The introduced impurities are diffused by heat treatment, to form an intrinsic base layer 7, external base layers 8 and an emitter layer 9.
FIG. 14 shows a cross-sectional structure of the transistor thus fabricated, and FIG. 15 shows an impurity profile (around the emitter layer 9) where the structure is taken along the line XX' of FIG. 14.
When the base layer 7 is made thin for high performance of the transistor, a high impurity concentration of the base layer 7 is required in order to ensure a collector-emitter breakdown voltage. In this case, the impurity concentration of the base layer 7 grows high around the emitter, particularly in a surface portion 7c, so that an emitter-base junction breakdown voltage decreases disadvantageously.